Fluidic circuits are used in a variety of applications where it is desired to measure some physical attribute by taking advantage of the correlation between information outputted from the circuit and the pressure or flow rate effects produced therein by the attribute in question. Typically, the circuit comprise a power source (i.e. a source of pressurized fluid such as a pump), at least one sensing element that responds to a change in the attribute (i.e. performs a data acquisition function in the circuit), and other elements that collectively respond to the fluidic output of the sensing element in order to provide information in a form which is suitable for downstream processing (i.e. perform a data conditioning function in the circuit). Output signals (i.e. pressures or flow rates) of a fluidic circuit may be used to directly actuate a control mechanism designed to respond to a change in hydraulic force, but are often used to drive a transducer so that the underlying information is provided in an electronic form more suitable for higher-level control. In the latter uses, the circuit typically comprises two fluidic oscillators used as pressure-to-frequency converters, each receiving as its input a fluidic signal originating in one of two receiving channels of the sensing element and communicated through one or more fluidic amplifiers, and each producing a fluidic wavetrain as its output. The frequency of the wavetrain depends on the magnitude of the fluidic input signal, and the data to be correlated with the pertinent physical attribute are differential frequency values of the wavetrains. The performance of such circuits is adversely affected by pressure-wave interference in vent passages connected to both oscillators and in flow passages through which the fluidic input signals are communicated to the oscillators. Over a given range of frequency differentials on either side of zero, this interference results in a "lock-on" phenomenon in which it appears that one wavetrain is effectvely stretched or contracted to match the frequency of the other. Consequently, resolution of values for the pertinent physical attribute is severely impaired over a range associated with the aforementioned range of frequency differentials. In addition, noise from each oscillator may be communicated upstream to the last-stage amplifier. This noise may adversely affect the fluidic signals outputted from the amplifier and the performance of the amplifier itself. Consequently, the quality and accuracy of the pressure signals inputted to the oscillators may be impaired.
The above-described problems are particularly pronounced in applications where the allowable space for the circuit is very limited, thus requiring that the oscillators and other circuit elements be positioned in close proximity to each other (e.g. fluidic angular rate sensing systems for guided missiles). Moreover, when the pump is of a type which provides an alternating flow rate, it may become an additional source of noise in such applications.
Publication HDL-TM-88-2 entitled "A Chemical Agent Calibrator Using Fluidic Oscillator Flowmeters" addresses the problem of oscillator noise in a fluidic circuit used in chromotography applications. The authors suggest that satisfactory results may be obtained by providing a fluidic low-pass filter in the feedback channel of a multi-stage oscillator, in combination with electronic filtering of the transducer output signals. It is asserted that fluidic filtering alone is inadequate due to loading constraints imposed by the fluidic circuitry (page 11, fourth paragraph). Interference between oscillators and potential interference from the source are not addressed.
The present invention exploits the discovery that in fluidic circuits of the type generally described above, a significant portion (estimated at 20-30 percent) of the noise created by operation of the oscillators manifests itself upstream from the oscillators. This and further discoveries in connection with the present invention provide for remarkably improved attenuation of oscillator-induced noise without sacrifice of overall circuit performance.
An objective of the invention is to improve resolution in fluidic measurement systems gradually, and in angular rate sensing systems in particular.
Another objective of the invention is to provide higher signal-to-noise ratios in fluidic circuits which employ oscillators.
These and such further objectives as are evidenced herein will be apparent from the following description, which includes the appended claims and accompanying drawings.